Tailoring and Identifying Brønsted Acid Sites on Metal Oxo-Clusters of Metal–Organic Frameworks for Catalytic Transformation

Metal–organic frameworks (MOFs) with Brønsted acidity are an alternative solid acid catalyst for many important chemical and fuel processes. However, the nature of the Brønsted acidity on the MOF’s metal cluster or center is underexplored. To design and optimize the acid strength and density in these MOFs, it is important to understand the origin of their acidity at the molecular level. In the present work, isoreticular MOFs, ZrNDI and HfNDI (NDI = N,N′-bis(5-isophthalate)naphthalenediimide), were prepared as a prototypical system to unravel and compare their Brønsted and Lewis acid sites through an array of spectroscopic, computational, and catalytic characterization techniques. With the aid of solid-state nuclear magnetic resonance and density functional calculations, Hf6 oxo-clusters on HfNDI are quantitatively proved to possess a higher density Brønsted acid site, while ZrNDI-based MOFs display stronger and higher-population Lewis acidity. HfNDI-based MOFs exhibit a superior catalytic performance in activating dihydroxyacetone (DHA) and converting DHA to ethyl lactate, with 71.1% selectivity at 54.7% conversion after 6 h. The turnover frequency of BAS-dominated Hf-MOF in DHA conversion is over 50 times higher than that of ZSM-5, a strong BAS-based zeolite. It is worth noting that HfNDI is reported for the first time in the literature, which is an alternative platform catalyst for biorefining and green chemistry. The present study furthermore highlights the uniqueness of Hf-based MOFs in this important biomass-to-chemical transformation.

Structural description and characterization of ZrNDI and HfNDI S2 S2.
Solid state nuclear magnetic resonance characterization of ZrNDI and HfNDI S4 S3 Density functional theory calculations S5 S4.
Catalytic performance of ZrNDI and HfNDI in DHA transformation reaction S9 S5.
Solution state NMR experiment S10
References S18    (8-x) ], M = Zr (a) or Hf (b), L = formate, x = 2, 3, 4, 5, or 6). The molecular representations of the most stable coordination configuration with a fixed quantity of water/hydroxyl groups in the Zr 6 and Hf6 oxo-cluster were also highlighted.   The overall partial density of d states is similar between Zr and Hf, and the band features don't change much after dehydrogenation and dehydration except for some subtle changes in the states near Fermi level. In the pristine oxo-cluster, the d states frontier of Zr cation is slightly lower in energy than that of Hf cation indicating higher stability of Zr cation. After dehydrogenation, the d states frontier of Hf cation falls behind Zr cation suggesting that Hf cation gets more stabilized than Zr cation during dehydrogenation, which explains stronger Bronsted acid site at Hf-O. In contrast, after dehydration, the energy difference between d states frontiers of Zr cation and Hf cation gets enlarged, which corresponds to the more destabilized dehydrated product of Hf oxo-cluster, and the weaker Lewis acid site at Hf. Figure S10. plots of the selectivity for PA conversion products for formic acid, ZrNDI, HfNDI, and blank test (no catalyst) after a 6h reaction in ethanol. The values of PA conversion (in percentage) of reactions are listed in black characters above each bar. Figure S11. PXRD patterns of ZrNDI and HfNDI soaked in ethanol (black), after a 6h DHA conversion reaction (red), and soaked in ethyl lactate (blue). There is no observable shift of PXRD peaks for samples confirming the integrity as well as the absence of pore expansion/contraction in the MOF acid catalyst. S10 Figure S12. Integral selectivity (in percentage) of pyruvaldehyde (PA, red), pyruvaldehyde ethyl acetal (PAEA, orange), pyruvaldehyde diethyl acetal (PADA, green), and ethyl lactate (EL, red) during the catalytic DHA transformation reaction using ZrNDI (a) or HfNDI (b). Error bars in the figures represent the standard derivation calculated from triplicate experiments. S11

S5. Solution state NMR experiment
To further illustrate the catalytic DHA-to-EL reaction process, we performed in situ 1 H NMR experiments to monitor the DHA-to-EL transformation reaction over HfNDI in deuterated ethanol (d6-EtOH) at 70 °C ( Figure S14a). The choice of lower reaction temperature for in situ NMR study is ascribed to the limitation of the NMR instrument in our lab. As shown in Figure S14b, after a 4h reaction at 70 °C, proton signals of the substrate (DHA) and products (PA, PAEA, and EL) are clearly visible. Based on the 1 H NMR data in Figure S14, the 1 H resonance peaks in Figure Figure S14c-5e), we successfully calculated the kinetics parameters of each chemical in the reaction ( Figure  S14f, S14g, and S13). In general, the 1 H NMR signal related to the PA and EL products progressively increased together accompanied with a gradual depletion of DHA over the experimental period. The product signal of PAEA (-CH, δ 1H = 5.0 ppm) was observed to gradually decrease over the experimental period ( Figure S13). As expected, no PADA side product was detected which corroborates well with the catalytic data shown in Figure 4. The selectivity of EL was then calculated to be approximately 71.4% (calculated based on the integration area in the 1 H NMR spectra measured after 4h reaction). This value is qualitatively consistent with the one reported in Figure 4b. The evolution rate for EL (based on δ 1H = 4.18 ppm) are calculated to be 2.5 times faster than PA (based on δ 1H = 8.03), with a value of ~0.006 min -1 ( Figure S13). The depletion rate of DHA is approximately 2-times faster with a value of ~0.012 min -1 . Figure S13. The expanded area of the in-situ 1 H NMR spectra in the region of 4.94 -5.06 ppm for the H f in PAEA. Blue and red lines represent the 1 H NMR spectra of the reaction at 0 (blue) and 4 h (red), respectively. The 1 H NMR spectra of the reaction mixture between 0 -4 h are shown in grey. Due to the weak 1 H NMR signal, attempts to integrate these peaks for kinetics study were unsuccessful. S12 Figure S14.   Figure S16. Proposed reaction pathway of the DHA-to-EL transformation reaction using HfNDI catalyst. S14

S6. Catalytic DHA-to-EL transformation reaction using Hf-MOF-808 and Hf-STA-26
To demonstrate the essential role of Hf 6 oxo-cluster in an efficient DHA-to-EL transformation reaction, we further explore the catalytic performance of another two Hf-based MOF, Hf-STA-26 3 and Hf-MOF-808. 4 Both Hf-STA-26 and Hf-MOF-808 possess intrinsic coordinatively unsaturated sites on their Hf 6 oxo-clusters. Hf-MOF-808 is constructed from six-connected Hf 6 oxo-clusters and three-connected 1,3,5-benzenetricarboxylate ligands. [4][5] Hf-STA-26 is a two-fold interpenetrated structure constructed from eight-connected Hf 6 oxo-clusters and three-connected CTTA ligand. 3,6 Hf-MOF-808 and Hf-STA-26 possess six and eight, respectively, intrinsic coordinatively unsaturated sites on each of their structural Hf 6 oxocluster. [3][4] Hf-STA-26 and Hf-MOF-808 were synthesized according to the literature using HfCl 4 as the metal salt and formic acid as modulator. 4,[6][7][8] It is worthy to note that the non-interpenetrated version of Hf-STA-26, Hf-NU-1200, 6 was reported by using benzoic acid as modulator. 3 The post-synthesis activating protocol for Hf-STA-26 and Hf-MOF-808 was analogous to that of HfNDI. PXRD measurements confirmed the successful synthesis of the targeted materials ( Figure S17b and S17d). The porosity of activated Hf-STA-26 and Hf-MOF-808 was examined by using 77 K N 2 sorption analysis. The values of BET surface area were calculated to be 1094 and 1572 m 2 g -1 for Hf-STA-26 and Hf-MOF-808, respectively, which are consistent to the reported values ( Figure S18). [4][5] The average pore sizes were determined to be 8. With the successful synthesis of Hf-STA-26 and Hf-MOF-808, we then examined the catalytic performance of these two samples in the application of DHA-to-EL transformation reaction. The protocol of catalytic performance test is analogous to that of ZrNDI and HfNDI instead of the use of Hf-STA-26 or Hf-MOF-808 as solid acid catalyst. The catalytic results were summarized and shown in Figure S19. A noteworthy result is that Hf-STA-26 can drive a complete DHA conversion within a 6-h reaction with an 84.1% EL selectivity ( Figure S19). The initial reaction for Hf-STA-26 was calculated to be 54.2 µmol g -1 min -1 , which is 7-times to that for HfNDI. The superior catalytic performance for Hf-STA-26, as compared to HfNDI, was attributed to a more opening pore structure in Hf-STA-26. The crystallographic pore aperture was estimated to be 6 Å for Hf-STA-26 3 ( Figure S17) and 3 x 9 Å in HfNDI ( Figure  S20). A more opening pore structure in MOF was reported to be beneficial to the substrate/product diffusion, resulting in a higher reaction rate. 9 Interestingly, even with the largest pore window (ø = 10 Å 4-5 , Figure S17) amongst the studied samples, Hf-MOF-808 drive the DHA conversion in a slowest rate (6.46 µmol g -1 min -1 for Hf-MOF-808 versus 8.78 µmol min -1 g -1 for HfNDI or 54.2 µmol min -1 g -1 for Hf-STA-26). Considering an envisioned better substrate/product diffusion in Hf-MOF-808, we attributed the observed slower reaction kinetics for Hf-MOF-808 to the difference of acidity composition (type/strength/density) in this material. Hf-MOF-808 is a (3,6)-connected framework with six coordinatively unsaturated sites on its Hf 6 oxo-cluster; while in HfNDI and Hf-STA-26, the number of coordinatively unsaturated sites are 8 and 4, respectively. In a potentiometric acid-base titration study, the S16 acidity and proton topology in Zr-or Hf-based MOFs were evaluated to be structuredependent. 10 A systematic study is ongoing in our labs to unveil the relationship between the number/position of the coordinatively unsaturated sites and the acidity properties (composition/strength/density) on a Hf 6 oxo-cluster.